Gyroscopes may be used to measure an angular change in direction. Conventional IFOGs employ, inter alia, a sensing coil to measure the rate of rotation of the gyroscope. The sensing coil of the IFOG typically includes an optical fiber wound on a mandrel or into a freestanding coil. Light is introduced to the rotating sensing coil and an optical splitter splits the light into two counter-propagating beams. One beam is propagated in the same direction as the rotating coil and the other beam is propagated in the direction opposite the direction of the rotating coil. Because light propagating in the same direction of rotation as the sensing coil travels a longer distance than the light propagating in the opposite direction of the sensing coil, a phase shift results when the counter propagating beams return to the optical splitter. A detector is used to measure the power of the counter-propagating beams of light which represents the phase shift between the two counter-propagating beams. The relative phase shift of the counter-propagating beams is indicative of the rate of rotation of the sensing coil about its axis. The fundamental measure of the precision of the rate of rotation of an IFOG is known as angle random walk (ARW), measured in degrees per square root hours (degree/√hr). The ability to measure power of the counter-propagating beams represents the sensitivity of the IFOG.
For a sensing coil with a fixed volume and low-loss optical fiber, the precision of an IFOG, as indicated by the ARW, is proportional to the square of the fiber diameter. This is because the ARW is inversely proportional to the length of the fiber in the sensing coil and the length of the fiber is inversely proportional to the square of the fiber diameter. Hence, the ARW of the sensing coil can be improved by using fiber with a reduced diameter and/or increased length. The ARW can also be improved by increasing the spectral width of the light source of the IFOG because the ARW of an IFOG is inversely proportional to the square root of the spectral width. Hence, increasing the spectral width of the light source reduces the ARW. However, the light source of a conventional IFOG is limited by the bandwidth of the fiber. A conventional optical fiber could be deigned to have a bandwidth of several hundred nanometers. However, increasing the bandwidth increases the bend loss in the fiber. In practice, when a conventional fiber is employed in an IFOG, the fiber is typically designed to have a bandwidth of less than 100 nm to minimize bend loss of the fiber.
One technique to improve sensitivity of an IFOG is to increase the number of turns of the optical fiber in the sensing coil thereby increasing the length of the optical fiber which increases the phase difference between the counter propagating beams of light in the sensing coil. Increasing the phase difference between the counter-propagating beams changes the power such that the ability of the detector to measure the power improves, thereby improving the sensitivity of the IFOG. However, increasing the number of turns of optical fiber in the sensing coil increases the size of the sensing coil, which increases package diameter of the IFOG. A typical prior IFOG manufactured with conventional optic fiber would have a package diameter of about 3 inches.
Typical conventional optical fibers employed in the sensing coils of prior IFOGs include a solid center core surrounded by a solid clad. The solid core is used to transmit light while the clad reflects the light within the core. Typically, the optical fiber is designed such that the index of refraction of the core (n1) is greater than the index of refraction of the clad (n2) thereby totally reflecting the light at the core-cladding interface. However, the conventional optical fibers utilized in the sensing coils of prior IFOGs have significant loss of light (bend loss) due to bending associated with winding the fiber into a sensing coil, which decreases both the accuracy and precision of the IFOG.
The majority of prior IFOG sensing coils are made with glass fibers that are made with silica (SiO2). For silica based fibers, the index difference needed to confine the light within the core is accomplished by either adding a dopant to the core of the fiber (up-doping) or adding a dopant to the cladding of the fiber (down-doping). For example, germanium or phosphorus may be added to the core of the fiber (up-doped) or fluorine or boron may be added to the cladding (down-doped).
One prior attempt to overcome the large bend loss associates with conventional optical fibers utilized in sensing coils of prior IFOGs is to increase the index difference between the core and the clad, e.g., the index difference between n1 and n2. Increasing the index difference decreases the bend loss because it increases the degree to which light is confined inside the core of fiber.
The most common fiber used in conventional IFOG sensing coils is a silica based fiber where the core is up-doped with germanium. The bend loss in the sensing coils made with these fibers can be decreased by increasing the amount of germanium in the core. However, there are two limitations to up-doping the core. First, the amount of index change that can be obtained by doping silica with germanium is less than ten percent. Second, because the material properties of doped silica are different from those of undoped silica, the manufacturing process used to make the fiber introduces more and more defects as the amount of germanium is increased. These defects increase the amount of light absorbed by the fiber per unit length.
Another drawback to conventional IFOG sensing coils made with germanium doped optical fiber is that this fiber can be damaged by gamma radiation. Most dopants, like germanium, increase the degree to which gamma radiation causes ionization in the fiber. This ionization increases the optical loss of the fiber and hence decreases the precision of the IFOG.
One prior art attempt to overcome the gamma radiation sensitivity and the length loss problems associated with germanium doped optical fibers is to dope the clad with fluorine to decrease the index of refraction of the clad n2 (e.g., increase the difference of the index of refraction between n1 and n2 by decreasing n2). Fluorine absorbs less light and less gamma radiation than germanium. However, there is a limit to the difference between the index of refraction between the core (n1) and the clad (n2), hence, fluorine doped fibers have limited ability to reduce bend loss.
Conventional optical fibers utilized in the sensing coil of prior IFOGs are also thermal and ionization radiation sensitive.